I am experiencing this same issue.  I'm also running PE11, current on updates, on Win 7 64bit.   My experience is that it's rare, but anoying.

I haven't experienced this in any of my other projects but the one I'm currently working on.  When I (re)open the project, it's fine, but once I start marking key frames and manipulating a clip, the mouse behaves as if I've right clicked on the timeline scroll bar and won't stop scrolling the timeline to the left and right unless I position the mouse in the center of the screen. 

I'll suggest it's not a video driver issue since I'm only experiencing this in one project, but I'd like to hear from others if there's some preference option that I've enabled thats causing this behavior, or if there's some way I can "reset" project to behave like my  others.

Regards,

Jim

Thanks for looking into this o promptly.  It's in the middle of the clip I'm marking.  It just happened again to me, and I'm paying a bit more attention to what happens when it occurs. 

All is well as I'm working on a video track,  using the "go to next/previous keymark" arrows, which moves the CTI.  I want to fade to black so I right click and hold to drop the marker, but when I release the right mouse button, the diamond marker is "stuck" and moves left/right/up/down as I move the mouse.  (unexpected behavior)  I have to right click the mouse a couple of times to get it to release the key frame marker.

Now when I move the mouse to the far left to select the "next keymark" arrow, the CTI stays put on the timeline and scrolls off the screen on the right as I move the mouse to the left, dragging the whole timeline back to the beginning.  The keyframe that I just dropped is somehow corrupt.  The CTI won't land there if I click next/previous and if I put the CTI on top of it and try to delete it, it stays on the screen.  Deleting the clip from the timeline and inserting it again (loosing my markers) returns me to a stable state.

I'm guessing it's a mouse driver.  My PC is a Dell laptop and I'm using a USB connected mouse and keyboard, rather than the touchpad.

It's been a week since I installed the drivers from my mouse vendor (Kensington), and I haven't had the problem.  Hope I don't jinx it with this post.